1. Field of the Invention
The present invention relates to an analog signal switching device for use in optical memory devices which record, playback, and erase information by irradiating the optical data storage medium with a light beam.
2. Description of the Prior Art
Conventionally, optical memory devices include a magneto-optical memory device which records information on a magneto-optical disk. Formed on the magneto-optical disk use in the magneto-optical memory device is a magnetic anisotropic magnetic thin film with an axis of easy magnetization in a direction perpendicular to the film surface, enabling information to be recorded according to the magnetic orientation in minute ranges.
When recording information on magneto-optical disks of this type, the magnetic thin film is irradiated with a focused laser beam with a diameter of, for example, approximately 1 micrometer. Thus, the temperature of that part of the thin film irradiated by the laser beam rises locally, coercive force drops, and the magnetic orientation of the film becomes more easily changed. At this point, it is possible to record or erase information through optical-thermal-magneto recording by changing the magnetic orientation to the desired orientation, for example, by simultaneously applying an external magnetic field to obtain reversed magnetic orientation.
Furthermore, when information recorded according to the above described manner is played back, the magnetic film is irradiated with a weak laser beam having a strength which will not cause the temperature to rise. Thus, the plane of polarization of the linear polarized light in the transmitted light and reflected light of the laser beam incident upon the magnetic thin film is rotated by the Faraday effect and the Kerr effect. Then, by the wave-detection of the transmitted light and reflected light using an analyzer, and also by converting the light to an electrical signal with an optical sensor, a pulse-type playback signal is obtained.
However, the optical sensor also receives the strong laser beam transmitted and reflected o the magnetic thin film during data recording or erasing. Moreover, because the laser beam emitted during recording or erasing is, as described above, a stronger laser beam when compared with that during playback, the optical sensor outputs an excessively high level signal.
When this excessively high level signal is input to a playback analog circuit connected to the optical sensor, the signal greatly exceeds the dynamic range, thus adversely affecting the subsequent playback operation.
Thus, as shown in FIG. 1, an analog signal switching device 10 has conventionally been used. Specifically, analog signal switching device 10 is so constructed that input terminal 12 is directly connected to terminal 11a on one side of analog switch 11, and input terminal 12 is connected to the other terminal 11b through sample hold circuit 13.
The analog switch 11 is so constructed that terminal 11a is connected to terminal 11c when the switch control signal SCS is low, and when the switch control signal SCS is high, terminal 11b is connected to terminal 11c. Furthermore, sample hold circuit 13 is so arranged as to enter a sampling state and output the signal sent from input terminal 12 unmodified when the switch control signal SCS is low, and when the switch control signal SCS is high, to enter a hold state to hold and output the signal sent from input terminal 12 immediately before the switch control signal SCS became high.
As shown in FIG. 2, by using an analog signal switching device 10 of such a type, when the switch control signal SCS is made high during the input of an excessively high level signal (which is several times the playback signal level), i.e., during information recording or erasing from input terminal 12, analog switch 11 produces from its terminal 11c a signal with the excessively high level signal removed.
Therefore, voltage generator 14 and comparator 15 connected to analog switch 11 operate normally, and a binary converted playback digital signal is output.
However, in the above described analog signal switching device 10, although it is possible to remove the excessively high level signal, a switching noise signal tends to be generated during switching of the analog switch. A noise signal of this type poses the problem that the noise signal often remains as a noise pulse even after the binary converted playback digital signal is generated by voltage generator 14 and comparator 15, thus causing improper operation of digital signal processing circuits, such as a demodulation circuit and a PLL (phase locked loop) circuit, and thus reducing the reliability of the optical memory device.